Diagnostic methods for selective catalytic reduction (scr) exhaust treatment system

ABSTRACT

In an internal combustion engine system having an exhaust aftertreatment system including a selective catalytic reduction (SCR) catalyst, diagnostic methods involve the intrusive perturbation of a target surface coverage parameter theta to determine the state of health of the SCR catalyst or an ammonia concentration sensor. An adaptive learning block adapts the target theta based on the use of NH 3  sensing feedback from a mid-brick positioned ammonia concentration sensor to pull in system variation. A further diagnostic monitors the amount of adaptation and when the adaptive learning excessively learns, the diagnostic assumes that some system-level degradation must have occurred and the diagnostic will notify the overall emissions control monitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 61/108,172 filed Oct. 24, 2008 entitled “DIAGNOSTIC METHODS FORSELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST TREATMENT SYSTEM and EXHAUSTGAS TREATMENT SYSTEM AND METHODS FOR OPERATING THE SAME” (attorneyDocket No. DP-318283), the disclosure of which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates generally to diagnostics and moreparticularly to diagnostic methods for selective catalytic reduction(SCR) based engine exhaust treatment systems.

BACKGROUND OF THE INVENTION

The relevant background includes the fields of exhaust gas treatmentsystems and diagnostics therefore. As to the former field of endeavor,there have been a variety of exhaust gas treatment systems developed inthe art to minimize emission of undesirable constituent components ofengine exhaust gas. It is known to reduce NOx emissions using a SCRcatalyst, treatment device that includes a catalyst and a system that isoperable to inject material such as ammonia (NH₃) into the exhaust gasfeedstream ahead of the catalyst. The SCR catalyst is constructed so asto promote the reduction of NOx by NH₃ (or other reductant, such asaqueous urea which undergoes decomposition in the exhaust to produceNH₃). NH₃ or urea selectively combine with NOx to form N₂ and H₂O in thepresence of the SCR catalyst, as described generally in U.S. PatentPublication 2007/0271908 entitled “ENGINE EXHAUST EMISSION CONTROLSYSTEM PROVIDING ON-BOARD AMMONIA GENERATION”. For diesel engines, forexample, selective catalytic reduction (SCR) of NOx with ammonia isperhaps the most selective and active reaction for the removal of NOx inthe presence of excess oxygen. The NH₃ source must be periodicallyreplenished and the injection of NH₃ into the SCR catalyst requiresprecise control. Overinjection may cause a release of NH₃ (“slip”) outof the tailpipe into the atmosphere, while underinjection may result ininadequate emissions reduction (i.e., inadequate NOx conversion to N₂and H₂O).

These systems have been amply demonstrated in the stationary catalyticapplications. For mobile applications where it is generally not possible(or at least not desirable) to use ammonia directly, urea-watersolutions have been proven to be suitable sources of ammonia in theexhaust gas stream. This has made SCR possible for a wide range ofvehicle applications.

Increasingly stringent demands for low tail pipe emissions of NOx havebeen placed on heavy duty diesel powered vehicles. Liquid urea dosingsystems with selective catalytic NOx reduction (SCR) technologies havebeen developed in the art that provide potentially viable solutions formeeting current and future diesel NOx emission standards around theworld. Ammonia emissions may also be set by regulation or simply as amatter of quality. For example, proposed future European emissionstandards (e.g., EU 6) for NH₃ slip targets specify 10 ppm average and30 ppm peak. However, the challenge described above remains, namely,that such treatment systems achieve maximum NOx reduction (i.e., atleast meeting NOx emissions criteria) while at the same time maintainingacceptable NH₃ emissions, particularly over the service life of thetreatment system.

In addition to the substantive emissions standards described above,vehicle-based engine and emission systems typically also require variousself-monitoring diagnostics to ensure tailpipe emissions compliance. Inthis regards, U.S. federal and state on-board diagnostic regulations(e.g., OBDII) require that certain emission-related systems on thevehicle be monitored, and that a vehicle operator be notified if thesystem is not functioning in a predetermined manner. Automotive vehicleelectronics therefore typically include a programmed diagnostic datamanager or the like service configured to receive reports fromdiagnostic algorithms/circuits concerning the operational status ofvarious components or systems and to set/reset various standardizeddiagnostic trouble codes (DTC) and/or otherwise generate an alert (e.g.,MIL). The intent of such diagnostics is to inform the operator whenperformance of a component and/or system has degraded to a level whereemissions performance may be affected and to provide information (e.g.,via the DTC) to facilitate remediation.

Over the service life of the above-described exhaust treatment systems,various constituent components can wear, degrade or the like, possiblyimpairing overall performance. For example, degradation of either theSCR catalyst or the dosing system may impair the treatment system inmeeting either or both of the NOx and NH₃ emission standards. Open loopcontrol does not appear to provide an adequate solution. It would beadvantageous to provide diagnostic routines to detect any suchdegradation.

There is therefore a need for diagnostic methods that minimize oreliminate one or more of the problems set forth above.

SUMMARY OF THE INVENTION

The invention has particular utility in an internal combustion engineincluding an exhaust gas treatment system having selective catalyticreduction (SCR) catalyst.

In one aspect of the invention, a method of performing a diagnostic isprovided. The method includes a number of steps. The first step includesintroducing a reductant (e.g., aqueous urea) into an exhaust gas streamin an amount based on a target surface coverage parameter theta. Thenext step involves perturbing the target theta parameter of inaccordance with a diagnostic function. Next, measuring an operatingcharacteristic of the exhaust gas treatment system. Finally, determininga state of health of a component of the treatment system based on anevaluation of the diagnostic function and the measured operatingcharacteristic.

In one embodiment, the component under diagnosis is the SCR catalyst.Through perturbation of the target theta about a nominal level, anexpected level of excess NH₃ is expected to be in the exhaust stream (atthe SCR catalyst). The NH₃ concentration level is measured as themeasured “operating characteristic”. A healthy SCR catalyst will exhibita relatively small magnitude perturbation in the NH₃ sensed feedback.However, for an SCR catalyst that has lost ammonia storage capability,the NH₃ sensed feedback exhibits a much larger magnitude, indicatingdegraded SCR catalyst performance.

In another embodiment, the state of health of an NH₃ sensor isdetermined. Likewise, if the measured NH3 sensing feedback tracks withthe target theta perturbation, then the NH₃ sensor is healthy.Otherwise, where the NH₃ concentration does not track the target thetaperturbation, the NH₃ sensor is unhealthy.

In another aspect of the invention, a diagnostic method is provided thatdetermines when NH₃ sensing feedback-based adaptive learning foradjusting the target theta values excessively learns. When thiscondition is detected, a fault or error is generated by the diagnostic.

A system is also presented.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example, withreference to the accompanying drawings:

FIG. 1 is a diagrammatic and block diagram showing an exhaust treatmentsystem in which the diagnostic methods of the invention may bepracticed.

FIG. 2 is a block diagram showing an overview of the dosing control thatincludes an SCR model as well as improvements in diagnostic methods.

FIG. 3 is a signal flow mechanization schematic showing inputs andoutputs of the SCR model.

FIG. 4 is a simplified diagram showing typical target theta (θ) valuesor curves as a function of temperature.

FIG. 5 is a flowchart showing a method of using theta perturbation fordiagnostics.

FIG. 6 is a combination chart showing a first embodiment of the methodof FIG. 5 involving theta perturbation for determining an SCR catalyststate of health.

FIG. 7 is a combination chart showing a second embodiment of the methodof FIG. 5 involving theta perturbation for determining an NH₃ sensorstate of health.

FIG. 8 is a flowchart showing a diagnostic method for generating a faultwhen theta adaptation exceeds control authority.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 1 is adiagrammatic and block diagram showing an exemplary diesel cycleinternal combustion engine 10 whose combustion exhaust gas 12 is fed toan exhaust gas treatment system 14. The exhaust gas is represented as astream flowing through the exhaust gas treatment system 14 and is shownas a series of arrows designated 12 _(EO) (engine out), 12 ₁, 12 ₂, 12 ₃and 12 _(TP) (tail pipe). It should be understood that while theinvention will be described in connection with an automotive vehicle(i.e., mobile) embodiment, the invention may find useful application instationary applications as well. In addition, embodiments of theinvention may be used in heavy-duty applications (e.g., highwaytractors, trucks and the like) as well as light-duty applications (e.g.,passenger cars). Moreover, embodiments of the invention may find furtheruseful application in various types of internal combustion engines, suchas compression-ignition (e.g., diesel) engines as well as spark-ignitionengines.

In the illustrative embodiment, the engine 10 may be a turbochargeddiesel engine. In a constructed embodiment, the engine 10 comprised aconventional 6.6-liter, 8-cylinder turbocharged diesel enginecommercially available under the DuraMax trade designation. It should beunderstood this is exemplary only.

FIG. 1 also shows an engine control unit (ECU) 16 configured to controlthe operation of the engine 10. The ECU 16 may comprise conventionalapparatus known generally in the art for such purpose. Generally, theECU 16 may include at least one microprocessor or other processing unit,associated memory devices such as read only memory (ROM) and randomaccess memory (RAM), a timing clock, input devices for monitoring inputfrom external analog and digital devices and controlling output devices.The ECU 16 is operable to monitor engine operating conditions and otherinputs (e.g., operator inputs) using the plurality of sensors and inputmechanisms, and control engine operations with the plurality of outputsystems and actuators, using pre-established algorithms and calibrationsthat integrate information from monitored conditions and inputs. Itshould be understood that many of the conventional sensors employed inan engine system have been omitted for clarity. The ECU 16 may beconfigured to calculate an exhaust mass air flow (MAF) parameter 20indicative of the mass air flow exiting engine 10.

The software algorithms and calibrations which are executed in the ECU16 may generally comprise conventional strategies known to those ofordinary skill in the art. Overall, in response to the various inputs,the ECU 16 develops the necessary outputs to control the throttle valveposition, fueling (fuel injector opening, duration and closing), spark(ignition timing) and other aspects, all as known in the art.

In addition to the control of the engine 10, the ECU 16 is alsotypically configured to perform various diagnostics. For this purpose,the ECU 16 may be configured to include a diagnostic data manager or thelike, a higher level service arranged to manage the reports receivedfrom various lower level diagnostic routines/circuits, and set or resetdiagnostic trouble code(s)/service codes, as well as activate orextinguish various alerts, all as known generally in the art. Forexample only, such a diagnostic data manager may be pre-configured suchthat certain non-continuous monitoring diagnostics require that suchdiagnostic fail twice before a diagnostic trouble code (DTC) is set anda malfunction indicator lamp (MIL) is illuminated. As shown in FIG. 1,the ECU 16 may be configured to set a corresponding diagnostic troublecode (DTC) 24 and/or generate an operator alert, such an illumination ofa MIL 26. Although not shown, in one embodiment, the ECU 16 may beconfigured so as to allow interrogation (e.g., by a skilled technician)for retrieval of such set DTCs. Generally, the process of storingdiagnostic trouble codes and subsequent interrogation and retrieval iswell known to one skilled in the art and will not be described in anyfurther detailed.

With continued reference to FIG. 1, the exhaust gas treatment system 14may include a diesel oxidation catalyst (DOC) 28, a diesel particulatefilter (DPF) 30, a dosing subsystem 32 including at least (i) areductant (e.g., urea-water solution) storage tank 34 and (ii) a dosingunit 36, and a selective catalytic reduction (SCR) catalyst 38. Inaddition, FIG. 1 shows various sensors disposed in and/or used by thetreatment system 14. These include a DOC inlet temperature sensor 39configured to generate a DOC inlet temperature signal 41 (T_(DOC-IN)), aNOx sensor 40 configured to generate a NOx signal 42 (NOx) indicative ofa sensed NOx concentration, a first exhaust gas temperature sensor 44,located at the inlet of the SCR catalyst 38, configured to generate afirst temperature signal 46 (T_(IN)), an optional second exhaust gastemperature sensor 48 configured to generate a second temperature signal50 (T_(OUT)), a first pressure sensor 52 configured to generate a firstpressure signal 54 (P_(IN)), a second pressure sensor 56 configured togenerate a second pressure signal 58 (P_(OUT)), and an ammonia (NH₃)concentration sensor 60 configured to generate an ammonia concentrationsignal 62 indicative of the sensed NH₃ concentration. In many commercialvehicles, a NOx sensor 64 is provided for generating a second NOx signal66 indicative of the NOx concentration exiting the tail pipe. However,such is shown for completeness only.

The DOC 28 and the DPF 30 may comprise conventional components toperform their known functions.

The dosing subsystem 32 is responsive to an NH₃ Request signal producedby a dosing control 80 and configured to deliver a NOx reducing agent atan injection node 68, which is introduced in the exhaust gas stream inaccurate, controlled doses 70 (e.g., mass per unit time). The reducingagent (“reductant”) may be, in general, (1) NH₃ gas or (2) a urea-watersolution containing a predetermined known concentration of urea. Thedosing unit 32 is shown in block form for clarity and may comprise anumber of sub-parts, including but not limited to a fluid deliverymechanism, which may include an integral pump or other source ofpressurized transport of the urea-water solution from the storage tank,a fluid regulation mechanism, such as an electronically controlledinjector, nozzle or the like (at node 68), and a programmed dosingcontrol unit. The dosing subsystem 32 may take various forms known inthe art and may comprise commercially available components.

The SCR catalyst 38 is configured to provide a mechanism to promote aselective reduction reaction between NOx, on the one hand, and areductant such as ammonia gas NH₃ (or aqueous urea, which decomposesinto ammonia, NH₃) on the other hand. The result of such a selectivereduction is, as described above in the Background, N₂ and H₂O. Ingeneral, the chemistry involved is well documented in the literature,well understood to those of ordinary skill in the art, and thus will notbe elaborated upon in any greater detail. In one embodiment, the SCRcatalyst 38 may comprise copper zeolite (Cu-zeolite) material, althoughother materials are known. See, for example, U.S. Pat. No. 6,576,587entitled “HIGH SURFACE AREA LEAN NOx CATALYST” issued to Labarge et al.,and U.S. Pat. No. 7,240,484 entitled “EXHAUST TREATMENT SYSTEMS ANDMETHODS FOR USING THE SAME” issued to Li et al., both owned by thecommon assignee of the present invention, and both hereby incorporatedby reference in their entirety. In addition, as shown, the SCR catalyst38 may be of multi-brick construction, including a plurality ofindividual bricks 381, 382 wherein each “brick” may be substantiallydisc-shaped. The “bricks” may be housed in a suitable enclosure, asknown.

The NOx concentration sensor 40 is located upstream of the injectionnode 68. The NOx sensor 40 is so located so as to avoid possibleinterference in the NOx sensing function due to the presence of NH₃ gas.The NOx sensor 40, however, may alternatively be located furtherupstream, between the DOC 28 and the DPF 30, or upstream of the DOC 28.In addition, the exhaust temperature is often referred to herein, andfor such purpose, the temperature reading from the SCR inlet temperaturesensor 44 (TIN) may be used.

The NH₃ sensor 60 may be located, in certain embodiments, at a mid-brickposition, as shown in solid line (i.e., located anywhere downstream ofthe inlet of the SCR catalyst 38 and upstream of the outlet of the SCRcatalyst 38). As illustrated, the NH₃ sensor 60 may be located atapproximately the center position. The mid-brick positioning issignificant. The sensed ammonia concentration level in this arrangement,even during nominal operation, is at a small yet detectable level ofmid-brick NH₃ slip, where the downstream NOx conversion with thisdetectable NH₃ can be assumed in the presence of the rear brick, evenfurther reducing NH₃ concentration levels at the tail pipe to withinacceptable levels. Alternatively, in certain embodiments, the NH₃ sensor60 may be located at the outlet of the SCR catalyst 38. The remainder ofthe sensors shown in FIG. 1 may comprise conventional components and beconfigured to perform in a conventional manner known to those ofordinary skill in the art.

The dosing control 80 is configured to generate the NH₃ Request signalthat is sent to the dosing unit 36, which represents the command for aspecified amount (e.g., mass rate) of reductant to be delivered to theexhaust gas stream. The dosing control 80 includes a plurality of inputsand outputs, designated 18, for interface with various sensors, othercontrol units, etc., as described herein. Although the dosing control 80is shown as a separate block, it should be understood that depending onthe particular arrangement, the functionality of (the dosing control 80may be implemented in a separate controller, incorporated into the ECU16, or incorporated, in whole or in part, in other control units alreadyexisting in the system (e.g., the dosing unit). Further, the dosingcontrol 80 may be configured to perform not only control functionsdescribed herein but perform the various diagnostics also describedherein as well. For such purpose, the dosing control 80 may includeconventional processing apparatus known in the art, capable of executingpre-programmed instructions stored in an associated memory, allperforming in accordance with the functionality described herein. Thatis, it is contemplated that the control and diagnostic processesdescribed herein will be programmed in a preferred embodiment, with theresulting software code being stored in the associated memory.Implementation of the invention, in software, in view of the foregoingenabling description, would require no more than routine application ofprogramming skills by one of ordinary skill in the art. Such a controlmay further be of the type having both ROM, RAM, a combination ofnon-volatile and volatile (modifiable) memory so that the software canbe stored and yet allow storage and processing of dynamically produceddata and/or signals.

FIG. 2 is a block diagram showing an overview of the dosing control 80of FIG. 1. The basic strategy is to control the dosing rate (e.g.,urea-water solution) so as to ensure that the there is adequate ammoniastored in the SCR catalyst 38 to achieve (i) a high NOx conversion rate(i.e., conversion of NOx into N₂ and H₂O), with (ii) a low occurrence orno occurrence at all of ammonia (NH₃) slips exceeding predeterminedmaximum thresholds.

Overall, the dosing control 80 is configured to generate an NH₃ Request,which is communicated to the dosing unit 36 (i.e., shown as the“NH₃/Urea Dosing”). In the illustrative embodiment, the NH₃ Request isindicative of the mass flow rate at which the dosing subsystem 32 is tointroduce the urea-water solution into the exhaust gas stream. Thecontrol variable used in implementing the dosing control strategy is aso-called ammonia surface coverage parameter theta (θ_(NH3)), whichcorresponds to the NH₃ surface storage fraction associated with the SCRcatalyst 38. In other words, the ammonia surface coverage parametertheta (θ_(NH3)) indicates the amount of ammonia—NH₃ stored in the SCRcatalyst 38. One aspect of the operation of the dosing control 80involves an SCR model 82.

FIG. 3 is a signal flow mechanization schematic showing inputs andoutputs of the SCR model 82. The SCR model 82 is a chemistry-based SCRmodel, and is shown with a theta control block 84, and a “NO and NO₂”predictor block 86. The SCR model 82 is configured to model the physicalSCR catalyst 38 and compute real time values for the ammonia surfacecoverage parameter theta (θ_(NH3)). The theta control block 84 isconfigured to compare the computed theta (θ_(NH3)) against a targetvalue for theta (“Target θ_(NH3)”), which results in a theta error. Thetheta control block 84 is configured to use a control strategy (e.g., aproportional-integral (PI) control algorithm) to adjust the requestedNH₃ dosing rate (“NH₃ Request”) to reduce the theta error. The thetacontrol block 84 also employs closed-loop feedback, being responsive toammonia sensing feedback by way of the ammonia sensor 60. The thetacontrol block 84 may use NH₃ feedback generally to adapt target thetavalues to account for catalyst degradation, urea injection malfunctionor dosing fluid concentration variation that may be encountered duringreal-world use. As will be described, the NH₃ sensing feedback is alsoused for various control and diagnostic improvements. The predictorblock 86 receives the DOC inlet temperature signal 41 (T_(DOC-IN)), theNOx sensor signal 42 and the exhaust flow signal 90 as inputs and isconfigured to produce data 88 indicative of the respective NO and NO₂concentration levels (engine out) produced by the engine 10. Thepredictor block 86 may comprise a look-up table (LUT) containing NO andNO₂ data experimentally measured from the engine 10.

The SCR model 82 may be configured to have access to a plurality ofsignals/parameters as needed to execute the predetermined calculationsneeded to model the catalyst 38. In the illustrative embodiment, thisaccess to sensor outputs and other data sources may be implemented overa vehicle network (not shown), but which may be a controller areanetwork (CAN) for certain vehicle embodiments. Alternatively, access tocertain information may be direct to the extent that the dosing control80 is integrated with the engine control function in the ECU 16. Itshould be understood that other variations are possible.

The SCR model 82 may comprise conventional models known in the art formodeling an SCR catalyst. In one embodiment, the SCR model 82 isresponsive to a number of inputs, including: (i) predicted NO and NO₂levels 88; (ii) an inlet NOx amount, which may be derived from the NOxindicative signal 42 (best shown in FIG. 1); (iii) an exhaust mass airflow (MAF) amount 90, which may be either a measured value or a valuecomputed by the ECU 16 and shown as exhaust MAF parameter 20 in FIG. 1;(iv) an SCR inlet temperature, which may be derived from the firsttemperature signal 46 (TIN); (v) an SCR inlet pressure, which may bederived from the first pressure signal 54 (PIN); and (vi) the actualamount of reductant (e.g., NH₃, urea-water solution shown as “NH₃Actual” in FIG. 2) introduced by the dosing subsystem 32. The actual NH₃amount helps ensure that the model provides accurate tracking of thereductant dosing. In one embodiment, values for theta (θ_(NH3)) areupdated at a frequency of 10 Hz, although it should be understood thisrate is exemplary only. There are a plurality of modeling approachesknown in the art for developing values for a surface coverage parametertheta (θ_(NH3)), for example as seen by reference to the article by M.Shost et. al, “Monitoring, Feedback and Control of Urea SCR DosingSystems for NOx Reduction: Utilizing an Embedded Model and AmmoniaSensing”, SAE Technical Paper Series 2008-01-1325.

Referring again to FIG. 2 the dosing control 80 includes additionalblocks. In particular, a target theta parameter (Target θ_(NH3)) block92 is shown, which is configured to provide a value for the target thetaparameter (Target θ_(NH3)) preferably as function of temperature (e.g.,exhaust gas temperature, such as the SCR inlet temperature TIN). Thetarget θ_(NH3), which is determined as a function of the SCR catalystinlet temperature T_(IN), is conventionally set-up based on thefollowing considerations: (1) desire to achieve a maximum possible NOxconversion efficiency with acceptable NH₃ slip levels (30 ppm peak, 10ppm average) for a given emission test cycle, and (2) recognition thatlimits must be set for the theta values at low temperatures to preventpotential high NH₃ slips upon sudden temperature ramp up in off-cycletests. In other words, in a pure ammonia storage control mode (i.e.,theta parameter control), different emission cycles may call fordifferent theta values in order to achieve the best NOx conversionwithin the confines of the applicable NH₃ slip limits.

FIG. 4 is a diagram showing exemplary target theta θ_(NH3) curvesdetermined for both the Euro Stationary Cycle (ESC) and the Federal TestProcedure (FTP) emission cycles using Cu-zeolite catalysts. As apractical matter, however, only one curve can be used in real worldsituations. The values from one of the target theta curves may be storedin a look-up table (LUT) or the like for run-time use by the thetacontrol block 84 of the dosing control 80. Such values may take the formof (temperature, theta value) data pairs.

As shown in FIG. 2, the theta control 84 further includes a comparator94 (e.g., a summer, or equivalent) configured to generate the thetaerror signal described above, indicative of the difference between thetarget theta (Target θ_(NH3)) and the computed theta (θ_(NH3)) from theSCR model. A PI control 96 is configured to produce an output signalconfigured to reduce the magnitude of the theta error. A high levelcontrol block 98 is responsive to various inputs to produce the NH₃Request signal, which is communicated to the dosing subsystem 32.

FIG. 2 also shows, in block form, a number of additional control anddiagnostic features. These additional control and diagnostic featuresmay be arranged to work together in some embodiments to achieve maximumNOx conversion while maintaining acceptable NH₃ slip levels undervarious driving conditions (i.e., in vehicle applications). The dosingcontrol 80 thus includes a number of functional blocks to implementthese features: a theta perturbation diagnostic block 100, an adaptivelearning diagnostic block 102, a transient compensation control block104 and an NH₃ slip control block 106.

The theta perturbation diagnostic block 100 is configured to perturb thetarget theta parameter in accordance with a small diagnostic functionand to measure the resulting response to determine the state of healthof one or more components of the exhaust treatment system 14.

The adaptive learning diagnostic block 102 includes a diagnostic featurethat monitors how much adaptation has been applied in adjusting thetarget theta parameter and generates an error when the level ofadaptation exceeds predetermined upper and lower limits. The logic inoperation is that at some level, the ability to adapt target thetavalues to overcome errors (e.g., reagent mis-dosing, reagent qualityproblems, SCR catalyst degradation) will reach its control limit formaintaining emissions. When this control limit is exceeded, thediagnostic generates an error.

The transient compensation block 104 is configured generally to reduceNH₃ dosing when specified exhaust transients are detected, such assudden increases in exhaust mass air flow or when an exhaust temperaturegradient is in an “increasing” state. The NH₃ slip control block 106 isconfigured to selectively shut-off NH₃ dosing when the measured NH₃ sliplevel (mid-brick sensor) exceeds a predetermined trip level at a timewhen certain other exhaust conditions are satisfied (e.g., temperaturegradient is in the “increasing” state). These features are described ingreater detail in co-pending patent application entitled “EXHAUST GASTREATMENT SYSTEM AND METHODS OF OPERATING THE SAME”, (Attorney DocketNo. DP-318318), filed on even date herewith, owned by the commonassignee of the present invention, the disclosure of which is herebyincorporated by reference in its entirety.

Theta Perturbation Diagnostics. FIG. 5 is a flowchart showing adiagnostic method involving theta perturbation to determine a state ofhealth of one or more different components of the exhaust treatmentsystem 14. As described in the Background, one challenge for SCR-basedexhaust treatment system developers is to incorporate sufficientrobustness in the control scheme to maintain performance throughout theservice life of the exhaust treatment system. However, the performanceof various components can degrade over time and with usage, even beyondthat correctable by a robust control scheme. Accordingly, it would bedesirable to know the state of health of one or more individualcomponents in order to take appropriate diagnostic or control action.For example, such diagnostic, upon determining a degradation of acomponent, may be configured to issue an alert to the operator (e.g.,such as turning on a MIL 26—FIG. 1) indicating that a component hasdegraded in performance to the point where it is not correctable by thecontrol system and hence may have emissions implications. Additionally,the diagnostic may further be configured to set a diagnostic troublecode (e.g., DTC 24—FIG. 1) to facilitate troubleshooting by atechnician. As an overview, FIG. 5 shows a flowchart describing thegeneral diagnostic method, while FIGS. 6 and 7 will be used to describeparticular applications of this general method to determine the state ofhealth of an SCR catalyst (FIG. 6) and an ammonia concentration sensor(FIG. 7). The general method includes steps 110, 112, 114 and 116.

The method begins in step 110. Step 110 involves introducing a reductant(e.g., ammonia gas or urea-water solution, as described above) into theexhaust stream in an amount based on the target ammonia surface coverageparameter theta (target θ_(NH3)). This basic control approach hasalready been described above in connection with FIGS. 2-4. In addition,it has been described above that the dosing control 80 utilizes acalculated theta parameter (feedforward control—θ_(NH3)) in conjunctionwith closed-loop control via ammonia concentration level as feedbackfrom ammonia sensor 60, preferably located at a mid-brick position ofthe catalyst 38. The method proceeds to step 112.

In step 112, the diagnostic method involves perturbing the target thetaparameter in accordance with a known, predetermined diagnostic function.Preferably, this is performed during steady-state engine operatingconditions so any observed variations in the sensed NH₃ concentrationlevel signal can be safely attributed to the perturbation. In thisregard, applying a known, intrusive theta perturbation about the nominaltarget theta parameter value can be expected to result in a predictableresponse, which response can be measured and later evaluated todetermine the state of health. The method then proceeds to step 114.

In step 114, the diagnostic method involves measuring an operatingcharacteristic, preferably of a component of or associated with theexhaust treatment system. In the particular SCR catalyst and NH₃ sensorembodiments to be described below, this step of the method involvesmeasuring the NH₃ concentration level sensor output. It should beunderstood, however, that other sensor outputs may be measured or otheroperating characteristics can form the basis for determining the stateof health. The method then proceeds to step 116.

In step 116, the diagnostic method involves determining the state ofhealth of the component based on an evaluation of both (i) the originaldiagnostic function which formed the basis for the theta perturbation,and (ii) the measured operation characteristic that results from thetheta perturbation (or that is the result of the theta perturbation).The evaluation may involve an assessment of the (i) the signal amplitudeor magnitude of the measured, resultant operating characteristic in viewof perturbing diagnostic function, as compared to expected levels; (ii)the relative phasing (or delay time) of the measured, resultantoperating characteristic compared to expected phasing; as well as (iii)frequency of perturbation switch between the measured, resultantoperating characteristic relative to the perturbing diagnostic function.In addition, it should be understood that while the illustrativeembodiments use a periodic perturbing function (e.g., triangle wave),other functions are possible, for example only, use of a step function.

Diagnostic for SCR Catalyst State of Health. FIG. 6 is a combinationchart showing responses of an ammonia concentration sensor to thetaperturbation where the responses indicate, respectively, a healthy andan unhealthy SCR catalyst. The X-axis shows time (seconds), while theY-axis at left shows ammonia concentration (ppm) and the Y-axis at rightshows the theta parameter value. The SCR catalyst can degrade over timeand with usage, particularly its ammonia storage capability. Todetermine whether the SCR catalyst may have become degraded in itsammonia storage capability, the theta perturbation block 100 (best shownin FIG. 3) is configured to perturb the target theta parameter (targetθ_(NH3)) such than an excess of ammonia would be expected to be presentand available to be sensed by the ammonia concentration sensor 60(located at the mid-brick position). One predicate condition beforeusing the NH₃ sensor to check the SCR catalyst is to first confirmproper operation of the NH₃ sensor itself. This predicate check may beperformed using conventional methods, or may be performed using thetheta perturbation method described below in connection with FIG. 7.Accordingly, upon confirmation that the ammonia sensor is functioningproperly, if the magnitude of the measured ammonia concentration exceedsan expected level, then this finding implies that the SCR catalyst haslost some of its ammonia storage capability. Furthermore, a reduction inNOx conversion efficiency would be expected. As described above,appropriate diagnostic action may be taken, such as issuing operatoralerts or setting diagnostic trouble codes. In addition, the diminishedammonia storage capability may be communicated to the dosing control 80,which may in turn be configured to use this information in its thetacontrol strategy.

FIG. 6 shows the results of a simulation conducted to show how SCRcatalyst degradation may be detected. In particular, the simulationmakes use of a six inch SCR catalyst, which for purposes of thesimulation was considered to provide the nominal amount (i.e., 100%) ofammonia storage. To simulate a loss in ammonia storage capability, athree inch SCR catalyst was used for comparison. The three-inch SCRcatalyst can be taken as indicative of the degraded performance for asix-inch SCR catalyst. In FIG. 6, the target ammonia surface coverageparameter theta (target θ_(NH3)), was perturbed in accordance with apredetermined, known diagnostic function, and is shown by referencenumeral 118. The selected diagnostic function may be a periodic signal,which oscillates above and below the nominal value for theta (targetθ_(NH3)) for the operating condition shown. In the illustrativeembodiment, the diagnostic function is a triangle waveform, althoughmany other types of waveforms may be used. The response of the ammoniaconcentration sensor, for the well performing catalyst, namely thesix-inch SCR catalyst, is shown as trace 120. As shown, the ammoniaconcentration sensor output signal indicates a low ammoniaconcentration, having a magnitude designated by reference numeral 122.The response of the ammonia sensor for the three-inch SCR catalyst(“degraded”) is shown as trace 124, which has a significantly greatermagnitude (designated 126) than that of the normal SCR catalyst (trace120). The increased magnitude can be attributed to a loss of ammoniastorage capability in the “degraded” SCR catalyst, which may inreal-world applications impact NOx emissions performance (tail pipe). Toquantify this evaluation, a maximum expected magnitude threshold may beestablished, which may be no less than the magnitude 122, for example.From this, a “good”/“bad” evaluation may be made. Alternatively, a morespecific measure of the degradation can be obtained by comparing themeasured magnitude with the expected magnitude and arriving at a ratio,percentage or other more descriptive measure of storage capability (orloss thereof, e.g., 90% of full capability).

Diagnostic For Ammonia Concentration Sensor State of Health. FIG. 7 is acombination chart showing respective responses of a healthy and anunhealthy NH₃ concentration sensor. The X-axis shows time (seconds),while the Y-axis at left shows ammonia concentration (ppm) and theY-axis at right shows the theta parameter value. In this embodiment, thetheta perturbation varies the amount of ammonia provided to the SCRcatalyst, and the valuation involves determining whether the ammoniasensor can detect the resulting variations in ammonia concentration. Theperformance of the ammonia sensor to properly detect changes in NH₃concentration will allow distinguishing a properly functioning ammoniasensor from an improperly functioning one, for either control purposesor for diagnostic purposes. Again, preferably, the diagnostic isperformed under substantially steady-state engine operating conditions(i.e., constant exhaust flow and temperature) so that any variation inthe sensor response can be properly attributed to just the thetaperturbation.

FIG. 7 shows a trace 128 of the target ammonia surface coverage thetaparameter (target θ_(NH3)) as varied by the theta perturbation block 100about a nominal value in accordance with a diagnostic function (e.g.,triangle wave in the illustrative embodiment). The diagnostic functionis configured and applied so that an excess of NH₃ is expected to bepresent in the SCR catalyst for detection by the ammonia concentrationsensor 60 (located at a mid-brick position). Trace 130 illustrates theresponse of a properly functioning ammonia concentration sensor whiletrace 132 shows the response of a malfunctioning (or degradedperformance) ammonia concentration sensor. When the ammonia sensor doesnot measure the predicted concentration level of ammonia in the exhaustgas stream, based on the perturbed target theta parameter (targetθ_(NH3)), then the diagnostic determines that an ammonia sensorperformance issue exists. Note, the measured operating characteristichere may include a magnitude, or may be a relative phase between thetheta perturbation and the response, or may be a trackingcharacteristic. As shown in FIG. 7, a point 134, on the perturbed targettheta parameter trace 128, which is a local maximum on the trace,results in a corresponding local maximum in the output of a properlyfunctioning NH₃ sensor, for example as shown at point 1342. Likewise, apoint 136, on the perturbed target theta parameter trace 128, which is alocal minimum on the trace, results in a corresponding local minimum inthe output of a properly functioning NH₃ sensor, for example as shown atpoint 1362. Note, that this correspondence in the phasing in absent inoutput of the degraded sensor, as shown in trace 132.

Diagnostic—Target Theta Adaptation Exceeds Control Authority. FIG. 8 isa flowchart showing a further diagnostic method to detect whenadaptation of the target theta map (i.e., values) exceeds the controlauthority limits for such adaptation. While developers of SCR-basedexhaust treatment systems face challenges in configuring systems thatmeet both NOx emission standards as well as NH₃ slip targets, suchchallenges are heightened when one considers the additional requirementof in-use compliance over the service life of the system. The adaptivelearning block 102 (FIG. 3) is generally configured to account for andinsert compensation into the target theta values for catalystdegradation, urea injection malfunction or dosing fluid concentrationvariation that may be encountered during real-world use. The adaptivelearning block 102 is responsive to the ammonia concentration signal 62produced by sensor 60, preferably located at a mid-brick position. Theadaptive learning block 102 is configured to use the NH₃ sensingfeedback to adapt (or adjust) the target theta parameter (Targetθ_(NH3)), thereby “pulling in” system variation that may occur in thefield. The ability of the adaptation block 102 to adjust for dosingerrors and the like enables some level of control authority to bring thesystem to near nominal levels. At some point, however, the ability toadapt target theta to overcome errors due to reagent mis-dosing into theexhaust gas stream, the quality of the reagent being introduced (e.g.,the aqueous urea concentration level) and the state of the SCR catalyst,will reach its control limit for maintaining emissions desiredperformance. Accordingly, when the adaptive learning block 102excessively learns (i.e., adapts the target theta parameter beyond ahigh or low threshold, the diagnostic assumes that some system-leveldegradation must have occurred, and the diagnostic may be configured tonotify the overall emissions control monitor.

FIG. 8 is a flowchart of the logic of the this diagnostic in the contextof a simplified adaptation scheme. The diagnostic is configured fordetecting and reporting excessive adaptation. The method begins in step138, where the most recent, measured ammonia (NH₃) concentration levelis provided by the mid-brick positioned sensor, and which will be usedas feedback in the adaptation. The method then proceeds to step 140.

In step 140, the method is configured to determine whether the ammoniaconcentration level sensed by the ammonia sensor 60 (mid-brick) exceedspredetermined bounds. Predetermined bounds may be a defined windowextending higher (“upper bound”) and lower (“lower bound”) than theexpected ammonia concentration, to account for small variations that aredeemed not significant enough to require adaptation of the target thetaparameter (Target θ_(NH3)). If the answer in this decision step is “NO”then the method branches to step 142 (“Keep theta values”). Step 142means that the dosing control 80 will follow the existing target θ_(NH3)values (e.g., as shown in curve form in FIG. 4), but as modified by anyprevious adaptations. Otherwise, if the answer is “YES” (i.e., themeasured NH₃ concentration is out of bounds-outside the window definedabove), then the method branches to step 144. Following step 144 meansthat adaptation may be possible.

In step 144, the method is configured to determine whether the measuredNH₃ concentration is lower than the lower bound described above. If theanswer is “YES”, then the method branches to step 146.

In step 146, the adaptation method is configured to increase thethen-existing Target θ_(NH3) values by a predetermined step (e.g., acompensation factor>1). The method then proceeds to step 148.

In step 148, the method determines whether the compensation factor (asincreased) is still within an upper limit (i.e., does not exceed theupper limit). If the answer is “YES”, then the adaptation has notexceeded its control authority and the method returns to the beginning.However, if the answer in decision block 148 is “NO” then the methodbranches to step 150 (“Generate an error”). In this situation, the net,accumulated positive-going adjustment to the target theta due to theadaptation logic has exceeded the control authority limit. As alluded toabove, the control authority limits (both upper and lower) may beselected such that if exceeded, the logic can infer than there has beena compromise in one or more of the components of the exhaust treatmentsystem 14. These compromises in performance may be due to problems inthe dosing delivery, quality issues with the urea-water solution, orperhaps a decrease in the ammonia storage capability of the SCRcatalyst. In step 150, the diagnostic may set a diagnostic trouble code,may activate an alert to an operator, or take such other action may beappropriate.

Otherwise, in step 144, if the measured ammonia concentration exceedsthe upper bounds, thereby requiring adaptation, then the method branchesto step 152.

In step 152, the method is configured to decrease the then-existingtarget θ_(NH3) values by a predetermined step (e.g., a compensationfactor<1). The method then proceeds to step 154.

In step 154, the method determines whether the compensation factor (asdecreased) is still within the lower limit (i.e., is still higher thanthe lower limit). If the answer is “YES”, then the adaptation has notexceeded its control authority and the method returns to the beginning.However, if the answer in decision block 154 is “NO” then the methodbranches to step 150 (“Generate an error”; See the description of step150 above).

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. In an internal combustion engine having an exhaust treatment systemhaving a selective catalytic reduction (SCR) catalyst, a method ofperforming a diagnostic on the exhaust treatment system, comprising thesteps of: introducing a reductant into an exhaust gas stream in anamount based on at least a target surface coverage parameter theta (θ);perturbing theta in accordance with a diagnostic function; measuring anoperating characteristic of the treatment system; determining a state ofhealth of a component of the treatment system based on the diagnosticfunction and the measured operating characteristic.
 2. The method ofclaim 1 wherein said reductant is selected from the group comprisingammonia (NH₃) and urea, said measuring step including the substep ofmeasuring an ammonia concentration level.
 3. The method of claim 2wherein said one component comprises the SCR catalyst, said state ofhealth including an ammonia storage capability of the SCR catalyst, saidstep of determining the state of health including the sub-steps of:comparing the measured ammonia concentration level with a predeterminedthreshold; determining a state of health fault based on an amount thatthe measured ammonia concentration exceeds the predetermined threshold.4. The method of claim 3 further including the step of setting an SCRcatalyst fault.
 5. The method of claim 3 wherein said diagnosticfunction comprises one selected from the group (including a periodicfunction and a non-periodic function) a periodic function, said step ofdetermining the state of health further includes the substep ofdetermining whether the measured ammonia concentration level correlatesto the diagnostic function.
 6. The method of claim 3 further comprisingthe steps of: providing an ammonia concentration sensor for measuringammonia concentration; and verifying proper operation of the ammoniaconcentration sensor.
 7. The method of claim 6 further comprising thestep of: positioning the ammonia concentration sensor in a sensinglocation selected from the group comprising a mid-brick position of theSCR catalyst and a rear-brick position of the SCR catalyst.
 8. Themethod of claim 5 wherein said theta parameter is controlled inaccordance with a control strategy configured to increase NOx conversionin the SCR catalyst and reduce NH₃ emission from the SCR catalyst, saiddiagnostic function being configured to result in a detectable excess ofNH₃ emission from the SCR catalyst.
 9. The method of claim 2 whereinsaid determining a state of health step includes the sub-step of:comparing an aspect of the measured operating characteristic and thediagnostic function wherein the aspect is selected from the groupcomprising (i) a signal amplitude; (ii) a phase or time delay; and (iii)a frequency difference.
 10. The method of claim 2 wherein said onecomponent comprises an ammonia concentration sensor, said diagnosticfunction comprising a periodic function, said step of determining thestate of health including the sub-steps of: comparing a correlationfactor by comparing the measured ammonia concentration level withperiodic function; determining a state of health fault based on thecorrelation factor.
 11. The method of claim 10 further including thestep of setting an ammonia concentration sensor fault.
 12. The method ofclaim 11 further comprising the step of: positioning the ammoniaconcentration sensor in a sensing location selected from the groupcomprising a mid-brick position of the SCR catalyst and a rear-brickposition of the SCR catalyst.
 13. In an internal combustion enginehaving an exhaust treatment system having a selective catalyticreduction (SCR) catalyst, a method of performing a diagnostic on theexhaust treatment system, comprising the steps of: introducing areductant into an exhaust gas stream in an amount based on at least asurface coverage parameter theta (0) selected so as to increase NOxconversion and reduce NH₃ emission from the SCR catalyst; adapting thetheta parameter based on a measured NH₃ level emitted from the SCRcatalyst; generating an exhaust system fault when an adaptation amountfor the theta parameter exceeds a predetermined threshold.
 14. Themethod of claim 13 wherein said generating step includes the sub-stepsof: establishing respective upper and lower adaptation limits;generating the fault when the adaptations of the theta parameter exceedsone of the upper and lower adaptation limits.
 15. The method of claim 14wherein said reductant comprises one of ammonia and aqueous urea, saidstep adapting step includes the sub-steps of: defining a base value forthe theta parameter based on an inlet temperature of the SCR catalyst;determining a compensation factor based on a measured NH₃ concentrationlevel emitted from the SCR catalyst; and determining the adapted thetaparameter value in accordance with the base value and the compensationfactor.
 16. The method of claim 15 wherein said sub-step of determininga compensation factor includes the sub-step of: determining when themeasured NH₃ concentration level exceeds an upper bound and increasingthe compensation factor.
 17. The method of claim 16 wherein saidsub-step of determining a compensation factor includes the sub-step of:determining when the measured NH₃ concentration level is less than alower bound and decreasing the compensation factor.